The present invention relates to a pressure roller device for developing and fixing a formed image by applying a pressure thereto.
For example, Japanese Patent Laid-Open Publication No. 88739/1983 discloses a method in which an image is formed by using a media sheet that is coated with resinous microcapsules containing a photo-setting material, a colorless dye and an image receiving sheet that is coated with a developer for effecting a color development of the colorless dye. When an image forming light, such as light reflected from an original document, is irradiated onto the media sheet, the irradiated microcapsules are set by a reaction of the photo-setting material so that a latent image is formed on the media sheet through s partial setting of the microcapsules. Then, when a pressure is applied to the image receiving sheet and the media sheet is placed on each other, the unset microcapsules are ruptured. Therefore, the colorless dye flows out of the ruptured microcapsules so as to be subjected to color development by the developer of the image receiving sheet, such that a colored image corresponding to the image forming light is formed on the image receiving sheet.
The microcapsules have a minute diameter of about 5 .mu.m. A high linear pressure of about 90 kg/cm is required to rupture the unset microcapsules. Thus, conventionally, it has been so arranged that the media sheet and the image receiving sheet placed on each other are passed between pressure rollers so as to rupture the unset microcapsules. The quality of the colored image formed by the rupturing of the unset microcapsules is affected by the magnitude and uniformity of the pressure for rupturing the unset microcapsules, etc. In this connection, various pressure rollers have been designed. For example, in U.S. Pat. No. 4,343,234, uniform pressing force is obtained by deflecting each of a pair of rotary shafts for the pressure rollers. The pressing force is increased by employing a depression roller that is in pressing contact with one of the pressure rollers. However, in this known pressure roller device, since an area of pressing contact between the pressure rollers is large, a large pressure is required to be applied in order to increase a pressing force per unit area of pressing contact. Thus, problems arise that a pressing mechanism including a spring, etc. becomes large in size and expensive materials are required to be used.
Meanwhile, in Japanese Patent Laid-Open Publication No. 70253/1988, pressure rollers of a small diameter and at least one pair of backup rollers for pressing the pressure rollers are provided so as to increase the pressing force. However, in this prior art pressure roller device, an axial bending strength of the pressure rollers of a small diameter is low. Furthermore in the case where a plurality of pairs of backup rollers for pressing the pressure rollers are provided, it is difficult to uniformly maintain the pressing force of each of the backup rollers. Hence, it is difficult to uniformly press the media sheet and thus, strain is readily produced in an axial direction of the pressure rollers. In addition, the prior art pressure roller device has a drawback in that a pressing mechanism including a spring, etc. is required to be provided for each of a plurality of pairs of the backup rollers, thereby resulting in a rise of its production cost.
Meanwhile, when the pressure rollers have a diameter of 65 mm, a high linear pressure of about 90 kg/cm is required to rupture the unset microcapsules. In order to obtain this linear pressure in an overall region in the longitudinal direction of the pressure rollers, in the case where, for example, a sheet (A4-size of 21.times.29.7 cm) is fed sidewise, a load of 2673 kg (i.e., 90.times.29.7) is required to be applied to the pressure rollers. In order to apply such a large load to the pressure rollers, members for supporting the pressure rollers, for example, a frame, bearings, etc. must have a sufficiently large strength and a device itself for applying the load to the pressure rollers becomes large in size. Therefore, the known pressure roller device becomes large in size, resulting in an increase in its production cost. Furthermore, the known pressure roller device has many inconveniences, such as producing large noises from a rotational transmission mechanism which includes gears for transmitting rotation to the pressure rollers, etc. Moreover, the known pressure roller device is disadvantageous in that an operation for relieving pressure, in the event the of occurrence of a jam at the pressure rollers is quite troublesome. In addition, in the known pressure roller device, a problem arises that since the pressure rollers are required to have a large outside diameter and the pressure rollers are required to be bored in order to apply a uniform load to the pressure rollers, the production cost is increased.
In a pressure image forming device disclosed in Japanese Patent Laid-Open Publication No. 70253/1988 referred to above, the sheets are pressed between upper and lower pressure rollers (a pressing roller and a support roller) of a relatively small diameter, while a pair of upper backup rollers of a relatively large diameter and a pair of lower backup rollers of a relatively large diameter are, respectively, provided at sides of the upper and lower pressure rollers remote from a point of pressing contact therebetween and interpose the upper and lower pressure rollers along the axial direction of the upper and lower pressure rollers so as to urge the upper and lower pressure rollers, respectively by using a pressing mechanism for pressing the upper and lower backup rollers in the pressing directions of the upper and lower backup rollers. By this arrangement of the prior art pressure image forming device, the area of contact between the pressure rollers is reduced, so that the pressing force per unit area of contact between the pressure rollers is increased. Thus, a load applied to the pressing roller can be reduced. As a result, the frame, bearings, pressing mechanism, etc. can be made small in size, noises produced at the rotational transmission mechanism can be lessened and an operation for eliminating jamming of the pressure rollers can be easily performed.
However, in the prior art pressure image forming apparatus, since the pressure rollers are made small in diameter, the rigidity of the pressure rollers drops. Therefore, if a load is applied to opposite end portions of the backup rollers, central portions of the pressure rollers are deflected. Thus, a gap is produced between a pair of the pressure rollers at the central portions of the pressure rollers. Therefore, since a uniform pressure cannot be obtained along the longitudinal direction of the pressure rollers, a problem arises in that the colored image on the image receiving sheet has nonuniform density. In addition, the sheets are undesirably wrinkled by the deflection of the pressure rollers.
Furthermore, in the prior art pressure image forming apparatus, the upper backup rollers and the lower backup rollers interpose the upper and lower pressure rollers, respectively, so as to position the upper and lower pressure rollers. Thus, the upper backup rollers, for example, are disposed so as to form an angle of about 90.degree. relative to the center of the upper pressure roller and interpose the upper backup roller so as to press the upper pressure roller towards the lower pressure roller. Likewise, the lower backup rollers interpose the lower pressure roller so as to press the lower pressure roller towards the upper pressure roller. However, when a high pressure that is sufficient to rupture the unset microcapsules is to be obtained in this prior art pressure image forming apparatus of the above described arrangement the, diameter of the backup rollers should be made large so as to increase the pressing force of the backup rollers. As a result, it is difficult to reduce production costs of the prior art pressure image forming apparatus and make the prior art pressure image forming apparatus compact.